Global photovoltaic (PV) energy generation capacity grew fivefold to 35 gigawatts between 2007 and 2010, with 75% of the capacity available in Europe. Most PV technologies today are based on crystalline silicon (Si) wafers, with organic PVs largely being regarded as a far-in-the-future option. While silicon absorbs solar light effectively in most of the visible range (350-600 nanometers), it behaves poorly between 600-1,100 nm. In order to compensate for this weak absorption, most PV cells have Si wafer thicknesses between 200-300 nm, and are typically referred to as “optically thick” absorbers. In addition, a pyramidal surface texture is typically utilized in order to scatter incoming light over a wide range of angles, thus increasing the effective path length of the light cell.
However, these approaches have had a significant impact on the basic cost of PV cells as more materials and processing is required. Furthermore, for thick solar cells the photocarrier diffusion length is comparably short, and thus charge carriers generated away from the semiconductor junctions are not effectively collected. This has prevented PV technology from replacing conventional fossil fuel technologies for energy generation. Any technological development that could decrease the cost of PV cells by at least a factor of two would be a straightforward revolution in the industry. Such a development could be achieved by increasing the absorption efficiency of a solar cell, so that near-complete light absorption occurs along with photocarrier current collection.
Some techniques that utilize plasmonics have been investigated so far for increased efficiency, which are targeted towards creating thin-film solar cells with thicknesses 1-2 micrometers (μm). For example, by doping the semiconductor material with 20-100 nm diameter metallic nanoparticles, the particles can act as subwavelength scattering elements or near-field couplers for the incident solar radiation, increasing the effective scattering cross section.
Another method involves the coupling of incident solar radiation into surface plasmon polaritons (SPPs), which are electromagnetic waves that travel along the interfaces of metals and dielectrics. This SPP coupling can be achieved for example by corrugating the metallic back surface of the solar cell. In all these cases, one of the main challenges which remains is that the absorption in the semiconductor material needs to be higher than the plasmon losses in the metal. However, these losses become significant for solar wavelengths beyond 800 nm.
It should be emphasized that enhancing the absorption efficiency of weakly lossy materials offers a double advantage, as not only smaller quantities of absorbing materials can be used, but they can also be of inferior quality, thus in both cases reducing the overall cost of the device.
Some embodiments of the present disclosure relate to using metamaterials and metamaterial-based configurations to address these problems.
Metamaterials are artificially created materials that can achieve electromagnetic properties that do not occur naturally, such as negative index of refraction or electromagnetic cloaking. While the theoretical properties of metamaterials were first described in the 1960s, in the past 15 years there have been significant developments in the design, engineering and fabrication of such materials. A metamaterial typically consists of a multitude of unit cells, i.e. multiple individual elements (sometimes refer to as “meta-atoms”) that each has a size smaller than the wavelength of operation. These unit cells are microscopically built from conventional materials such as metals and dielectrics. However, their exact shape, geometry, size, orientation and arrangement can macroscopically affect light in an unconventional manner, such as creating resonances or unusual values for the macroscopic permittivity and permeability.
Some examples of available metamaterials are negative index metamaterials, chiral metamaterials, plasmonic metamaterials, photonic metamaterials, etc. Due to their sub wavelength nature, metamaterials that operate at microwave frequencies have a typical unit cell size of a few millimetres, while metamaterials operating at the visible part of the spectrum have a typical unit cell size of a few nanometres. Some metamaterials are also inherently resonant, i.e. they can strongly absorb light at certain narrow range of frequencies.
For conventional materials the electromagnetic parameters such as magnetic permeability and electric permittivity arise from the response of the atoms or molecules that make up the material to an electromagnetic wave being passed through. In the case of metamaterials, these electromagnetic properties are not determined at an atomic or molecular level. Instead these properties are determined by the selection and configuration of a collection of smaller objects that make up the metamaterial. Although such a collection of objects and their structure do not “look” at an atomic level like a conventional material, a metamaterial can nonetheless be designed so that an electromagnetic wave will pass through as if it were passing through a conventional material. Furthermore, because the properties of the metamaterial can be determined from the composition and structure of such small (nanoscale) objects, the electromagnetic properties of the metamaterial such as permittivity and permeability can be accurately tuned on a very small scale.
One particular sub-field of metamaterials are plasmonic materials, which support oscillations of electrical charges at the surfaces of metals at optical frequencies. For example, metals such as silver or gold naturally exhibit these oscillations, leading to negative permittivity at this frequency range, which can be harnessed to produce novel devices such as microscopes with nanometer-scale resolution, nanolenses, nanoantennas, and cloaking coatings.